Optimizing: Move optimization objects to Arena.

This should reduce the stack size needed by the
OptimizingCompiler::CompileOptimized() which was very
close to our limits for clang builds, causing repeated
build breakages on otherwise healthy changes:
    art/compiler/optimizing/optimizing_compiler.cc:395:37:
    error: stack frame size of 1760 bytes in function
    'art::OptimizingCompiler::CompileOptimized'
    [-Werror,-Wframe-larger-than=]

Change-Id: I2f4ab0235f4eac61823a4a320bb4fe78942a23c2